The invention relates to interconnection of electronic components, such as integrated circuit devices and other similar devices or burn-in and test structures, all of which may have an array of closely spaced conductive contact sites. More particularly the present invention provides a geometrical array of dual-fraction particulate contacts, distributed through flexible elastomeric sheets, for reliable, low force electrical interconnection between electronic components.
The introduction of solid-state semiconductor electronics provided the opportunity for progressive miniaturization of components and devices. One of the benefits of such miniaturization is the capability of packing more components into a given space. A drawback of miniaturization is the reduction in spacing between contacts on one device and the need for accurate alignment with corresponding contacts on a second device to provide reliable electrical interconnection between the two. Lack of planarity also affects interconnection of devices due to variation in the distance between the device contacts and an array of contacts intended to mate with the device contacts. Accurate engagement by some contacts leaves gaps between other contacts unless independent contacts have freedom to move across such gaps. Alternatively the connecting force between an array of contacts and device contacts increases to a level required for reliable interconnection, with potential for compression and damage to some of the contacts. Resilient anisotropically conductive interposers compensate for lack of device planarity to provide conductive pathways between electronic devices. Integrated circuit sockets used for performance testing, burn-in, and semi-permanent mounting to printed circuit boards employ various types of interposer during performance evaluation of finished, packaged, integrated circuit devices.
Interconnection of electronic components with finer and finer contact spacing or pitch has been addressed in numerous ways along with advancements in semiconductor device design. Introduction of ball grid array (BGA) devices placed emphasis on the need to provide connector elements with space between individual contacts at a minimum. One answer, found in U.S. Pat. No. 5,109,596 and U.S. Pat. No. 5,228,189, describes a device for electrically connecting contact points of a test specimen (circuit board) to the electrical contact points of a testing device using an adapter board having a plurality of contacts arranged on each side thereof. Cushion-like plugs made from an electrically conductive resilient material are provided on each of the contact points to equalize the height variations of the contact points of the test specimen. An adapter board is also provided made of a film-like material having inherent flexibility to equalize the height variations of the contact points of the test specimen. Furthermore, an adapter board is provided for cooperating with a grid made of an electrically insulated resilient material and having a plurality of plugs made from an electrically conductive resilient material extending therethrough. Successful use of this device requires accurate registration of contacts from the test specimen, through the three layers of planar connecting elements to the testing device.
U.S. Pat. No. 5,136,359 and U.S. Pat. No. 5,188,702 disclose both an article and a process for producing the article as an anisotropic conductive film comprising an insulating film having fine through-holes independently piercing the film in the thickness direction, each of the through-holes being filled with a metallic substance in such a manner that at least one end of each through-hole has a bump-like projection of the metallic substance having a bottom area larger than the opening of the through-hole. The metallic substance serving as a conducting path is prevented from falling off, and sufficient conductivity can be thus assured. While the bump-like projections of the anisotropic conductive films, previously described, represent generally rigid contacts, U.S. Pat. No. 4,571,542 and U.S. Pat. No. 5,672,978 describe the use of superposed elastic sheets over a printed wiring board, to be tested, and thereafter applying pressure to produce electroconductive portions in the elastic sheet corresponding to the contact pattern on the wiring board under test. In another example of a resilient anisotropic electroconductive sheet, U.S. Pat. No. 4,209,481 describes a non-electroconductive elastomer with patterned groupings of wires, electrically insulated from each other, providing conductive pathways through the thickness of the elastomer. Other known forms of interconnect structure may be reviewed by reference to United States Patents including U.S. Pat. No. 5,599,193, U.S. Pat. No. 5,600,099, U.S. Pat. No. 5,049,085, U.S. Pat. No. 5,876,215, U.S. Pat. No. 5,890,915 and related patents.
Commercial devices require some downward pressure to provide electrical connection between a device under test (DUT) and a printed circuit board or test socket attached to a load board. For this reason, conductive pathways are preferably short for optimum conductivity and ease of activation using a force sufficiently low to prevent damage to solder balls on a DUT. Reduction in activation force benefits testing at elevated temperatures where there is increased potential for damage to ball grid arrays.
In addition to the problem, mentioned previously, of interconnection failure caused by gaps between contacts, interconnection failure may occur by occlusion of a metal contact due to surface contamination with e.g. grease, non-conducting particles or a layer of metal oxide. Such an oxide layer results from air oxidation of the metal. Since oxide layers generally impede the passage of electrical current, reliable contact requires removal or penetration of the oxide layer as part of the interconnection process. Several means for oxide layer penetration, towards reliable electrical connection, may be referred to as particle interconnect methods as provided in U.S. Pat. No. 5,083,697, U.S. Pat. No. 5,430,614, U.S. Pat. No. 5,835,359 and related patents. A commercial interconnect product, described as a Metallized Particle Interconnect or MPI, is available from Thomas and Betts Corporation. The product is a high temperature, flexible, conductive polymeric interconnect which incorporates piercing and indenting particles to facilitate penetration of oxides on mating surfaces. Another commercial, electronic device interconnection product, available from Tecknit of Cranford, N.J., uses xe2x80x9cHard Hatxe2x80x9d and xe2x80x9cFuzz Buttonxe2x80x9d contacts in selected arrays. U.S. Pat. No. 4,574,331, U.S. Pat. No. 4,581,679 and U.S. Pat. No. 5,007,841 also refer to the xe2x80x9cFuzz Buttonxe2x80x9d type of contact.
The previous discussion shows that interconnection of electronic devices has been an area subject to multiple concepts and much product development in response to the challenges associated with mechanical issues of interconnection and resultant electrical measurements. Regardless of advancements made, there is continuing need for improvement in three key areas, namely registration between interconnecting devices and electronic components, flexibility of contact sets for reliable device interconnection and minimization of the force required for reliable interconnection with low contact resistance. In view of the continuing needs, associated with interconnect structures, the present invention has been developed to alleviate drawbacks and provide the benefits described below in further detail.
The present invention provides an interposer for a compliant interconnect assembly that may be used for reliable electrical connection between electronic devices at lower contact forces than other types of particle interconnect structures. An interposer according to the present invention comprises an elastomeric sheet having holes formed in it to accommodate conductive particles that are usually held together using a resilient binder. Conductive particles, selected for their size and shape, form conductive columns that provide electrical pathways through the elastomeric sheet. The arrangement of conductive columns in the elastomer produces an anisotropic or z-axis conductive sheet. Individual conducting columns have low contact resistance and respond to low compression force when used in a compliant interconnect assembly that connects a semiconductor device for testing. Compliant interconnect assemblies according to the present invention operate at low contact forces, e.g. as low as 12 g, while sustaining a large number of actuation cycles including effective cyclic actuation over a temperature range from  less than 0xc2x0 C. to about 125xc2x0 C. Other applications of a compliant interconnect assembly, according to the present invention, include use as a contactor, a production socket, a burn-in socket, a probing device, a board to board interconnect, as a device to device interconnect and similar applications.
Anisotropic interposers included in a compliant interconnect assembly are part of a low profile, rugged structure designed for solderless, releasable and remateable connections to delicate IC packages using low actuation forces. Repeated cycling between connect and disconnect configurations, over extended time periods, reveals the stability of contact resistance and reliable electrical performance of electrically conductive particle columns according to the present invention. Preferably, particles used to form the conductive paths through elastomeric sheets have substantially the same size or fall in one or more relatively narrow and substantially uniform size distributions. Particle size and shape affect contact resistance. Reduction in contact resistance usually occurs as particle size increases and the shape approaches spherical or at least spheroidal.
A single size distribution of relatively large particles in electrically conducting columns has a disadvantage of displacement and loss of individual particles during multiple connect/disconnect cycles. This may occur even in the presence of particle binders. Any loss of conductive particles changes the geometrical condition of the connecting surface of a conducting column. Changing geometry may interfere with contact reliability, resulting in an increase in contact resistance. A conductive column including a dual-fraction particle structure overcomes the problem of particle loss. Suitable conductive columns have a first conductive particle fraction or average size distribution covered, at least at one end, by a second conductive particle fraction having an average particle size distribution that is significantly smaller than the first. The smaller particle size material provides a cap over one or both ends of a central pillar that includes one or more layers of the particles of larger particle size. A binder holds together conducting columns according to the present invention having a dual-fraction particle structure. Cap forming particles have a size distribution from about 6 xcexcm to about 40 xcexcm, while the central pillar includes particles ranging in size from about 80 xcexcm to about 180 xcexcm.
To achieve low contact resistance, the application of force to compress a conductive column preferably acts to align particles in the direction of the longitudinal axis of the column. Lateral movement of particles interferes with inter-particle contact and increases contact resistance. There is a relationship between contact resistance and the hardness of the elastomer surrounding each dual-fraction particle column. As the hardness of the elastomer increases the contact resistance of conductive columns, in an interposer sheet, tends to decrease. While not wishing to be bound by theory, a harder elastomer restricts lateral movement of particles to favor alignment in the desired direction parallel to the longitudinal axis of the column. Particle alignment may be further enhanced if the particles have a substantially spherical shape.
More particularly, the present invention provides a conductive, Z-axis, particulate connector comprising an electrically conducting elastic column including a central pillar of conductive spheroidal particles having a first average particle size. The central pillar has a first end opposite a second end and a particulate cap extending from at least the first end of the central pillar. A particulate cap includes particle layer having a second average particle size. The second average particle size is less than the first average particle size. A cured elastomer binds the spheroidal particles and the particle layer to form the electrically conducting elastic column.
The present invention also provides a compressible interconnect comprising an electrically insulating elastomer sheet and at least one electrically conducting elastic column occupying a hole formed in the elastomer sheet, to provide a localized conductive path through the thickness of the insulating elastomer sheet. The at least one electrically conducting elastic column has a central pillar of conductive spheroidal particles of a first average particle size. The central pillar further has a first end opposite a second end, at least the first end contacting a particulate cap including particles having a second average particle size, which particle size is less than the first average particle size.
In addition, the present invention provides a compliant interconnect assembly to electrically connect a first electronic device to a second electronic device. The compliant interconnect assembly comprises a contact set including an electrically insulating flexible film having at least one conductive contact suspended therein. The interconnect assembly also includes a compressible interposer as an electrically insulating elastomer sheet matrix for at least one electrically conducting elastic column to provide a localized conductive path through the thickness of the elastomer sheet. The electrically conducting elastic column comprises a central pillar of conductive spheroidal particles having a first average particle size. The central pillar has a first end opposite a second end. At least the first end has a particulate cap bonded to it including particles having a second average particle size that is less than the first average particle size. The compliant interconnect assembly is formed when the contact set lies adjacent to the compressible interposer to provide engagement between the at least one conductive contact and the at least one conducting elastic column for electrically connecting the first electronic device and the second electronic device abutting opposite sides of the compliant interconnect assembly.
Terms used herein have the meanings indicated as follows:
The term xe2x80x9ccompressible interposerxe2x80x9d or xe2x80x9cflexible interposerxe2x80x9d or similar terms refers to a z-axis conducting structure formed from an elastomeric sheet having through-holes or vias to accommodate columns of conductive material to provide electrical current-carrying pathways through the elastomer sheet. An interposer forms part of a compliant interconnect assembly according to the present invention.
A xe2x80x9ccompressible interconnectxe2x80x9d or xe2x80x9cflexible interconnectxe2x80x9d or the like, as used herein, identifies a z-axis conducting structure formed from an elastomeric sheet having through-holes or vias to accommodate columns of conductive material to provide electrical current-carrying pathways that provide direct connection between electronic devices or electronic devices and printed circuit boards or load boards or the like.
As used herein xe2x80x9cconducting elastic columnxe2x80x9d or xe2x80x9cconductive columnxe2x80x9d or like terms refer to an electrically conducting channel providing an isolated current carrying pathway through a hole passing through the thickness of an elastomer sheet matrix. A conducting elastic column, being electrically isolated from its nearest neighbors, forms part of a z-axis conducting structure.
The terms xe2x80x9ccentral pillarxe2x80x9d or xe2x80x9cmid-pillarxe2x80x9d or xe2x80x9cpillar portionxe2x80x9d may be used interchangeably to identify a portion of a conducting elastic column that includes spheroidal particles.
The use of the term xe2x80x9cspheroidalxe2x80x9d relates to substantially symmetrical particles having a shape including rounded surfaces. A uniform sphere is the most symmetrical for spheroidal particle.
xe2x80x9cActuation forcexe2x80x9d refers to the average weight applied to provide reliable electrical connection in a pathway that includes a conducting elastic column.
xe2x80x9cContact resistancexe2x80x9d refers to the electrical resistance of the electrical current conducting pathway, between interconnected devices, during application of an actuation force to a compliant interconnection assembly, or compressible interconnect according to the present invention.
A record of the xe2x80x9cnumber of actuationsxe2x80x9d indicates the number of make and break connections that may be made, using a compliant interconnect assembly or compressible interconnect, during device burn-in and testing without exceeding a desirable level of actuation force and contact resistance.